The Na‐Ca and Na‐Mg ion‐exchange equilibria were studied in two soils and in an ion‐exchange resin by batch equilibration procedures. The apparent equilibria, in terms of concentrations, were different for the anions chloride or sulfate. The differences were reduced but not eliminated by ion activity corrections based on ionic strength. A more satisfactory agreement between the results in chloride and sulfate systems was obtained when allowance was made for solution phase activities and ion‐association of Ca and Mg with SO4 ions.